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An ellipsometric study of polymeric thionine films on platinum
J. Electroanal. Chem., 195 (1985) 189-196 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
189
Preliminary note AN ELLIPSOMETRIC STUDY OF POLYMERIC THIONINE FILMS ON PLATINUM
ANDREW HAMNETT Inorganic Chemistry Laboratory,
South Parks Road, Oxford (Great Britain)
A. ROBERT HILLMAN School of Chemistry, University of Bristol, Cantock’s Close, Bristol (Great Britain) (Received 15th July 1985; in revised form 14th August 1985) INTRODUCTION
The modification of electrode surfaces with polymeric redox films has received considerable attention recently [ 1,2]. Applications to electrocatalysis [ 31, energy conversion [ 41, semiconductor protection [ 51, display devices [ 61 and sensors [ 71 have been described. Characterisation has included electrochemical (cyclic voltarnmetry [8], chronoamperometry [9], chronocoulometry [lo], chronopotentiometry [ 111, normal pulse voltammetry [ 121 and rotating disc electrode [ 131) as well as spectroscopic (UV/visible [ 141, infrared [ 151, Rarnan [16], XPS [ 171, AES [18] and SIMS [19] ) techniques. However, not one of this armoury of techniques permits unambiguous in situ determination of film thickness: this is the question we address here. For example, integration of slow scan rate cyclic voltammograms allows estimation of the total (electroactive) coverage of redox groups, P/m01 cm -‘, but division of this into the concentration, c, and film thickness, L, is not obvious, particularly when swelling factors of up to the order of lo2 have been claimed [20]. Potential step measurements lead to a combination of the effective charge transport diffusion coefficient, D, and the concentration, in the form D “‘c; this essentially gives a relaxation rate constant, D/L’, as P is equal to the product of c and L and is known. Turning to the spectroscopic techniques, UV/visible absorbance data give r (if the extinction coefficient is known) for comparison with the electrochemical estimate, but the problem of the separation of c and L remains. XPS estimation of film thickness using attenuation of substrate photoelectron intensity [ 171 is, of necessity, ex situ, so the value of L does not relate to the electrochemical environment. Physical methods which have been used include step profiling [ 111 and simple micrometer measurement [20], but these are ex situ, are limited (severely in the latter case) in the range of measurable values and may result in film damage or distortion during measurement [20]. The importance of the concentration of redox centres, for which an absolute value is only obtainable if L can be determined, is of both fundamental interest and practical importance. Firstly, the rate of charge transport through the film may be dependent on the site-site separation, i.e. concentration - interestingly in either direction [ 10,211. Secondly, full exploitation of mediated charge 0022-0728/85/$03.30
o 1985 Elsevier Sequoia S.A.
190
transfer-based applications requires a knowledge of immobilised redox centre concentration. The usefulness of a given total loading of redox sites may depend crucially on their distribution, in that there is a trade-off between the rapid charge-transfer/mediation reaction kinetics and poor target solution species permeation/diffusion in compact films with high concentrations of mediator. Only those sites which are readily accessible are effective; further increase in film thickness merely impedes the transport processes and may result in the surface or surface reaction layer cases Sk”, St,, LSk or LSt, (in the notation of Albery [22] ) or R, E or SR (in the notation of Saveant [23] ). For a given value of c, there‘is an optimum value of L for efficient mediation, and the strategy for its determination has been delineated by Albery [22]. It is apparent, then, that L is an important parameter, but that techniques for its direct, in situ, unambiguous, non-destructive measurement are not well developed. The object of this preliminary note is to describe ellipsometric measurements of the thickness and optical properties of a polymeric thionine film [24] on a platinum electrode, firstly, during its growth by anodig electropolymerisation and, secondly, as a function of oxidation state in background electrolyte solution. Specifically, questions we set out to answer were: (i) is PtO formed (as it would be in the background electrolyte at these potentials); (ii) if PtO is formed, is it reduced to Pt at the less anodic potentials used to study the polymer film by cyclic voltammetry; (iii) what is the final film thickness; (iv) what is the growth law; (v) do the optical constants of the polymer differ from those of the monomer; and (vi) is the film thickness redox state dependent? In order to answer these questions we have also obtained results for PtO formation on Pt and for the reversibly (Langmuirian) adsorbed monomeric thionine (Th) species and its reduced form leucothionine (Leu). These will be described in detail in the full paper, and we simply use the results here. EXPERIMENTAL
The ellipsometer was a Rudolph Instruments RR2000 automatic ellipsometer with quartz optics and with a rotating analyser/fixed polariser configuration. No compensator was employed, and intensity, azimuthal angle, cu,and ellipticity, e, could be directly obtained from the analogue computer employed. From Q and E the ellipsometric angles A and \k could be calculated. The instrument was interfaced to an Intertec Superbrain, using digitised output from the intensity, azimuth and ellipticity readouts as described elsewhere [ 251. The period of the rotating analyser is 20 ms and the measurements reported here were made by averaging over a 0.2 s interval. The angle of incidence was 61.33” and the polar&&ion angle employed was 45” throughout. Light sources used were a Hughes 5 mW He-Ne laser with neutral density filter and an Ealing 75 W High Pressure Xe lamp with interference filter, for measurements at 632.8 and 450 rim, respectively.
191
The working electrodes were Pt sputtered on glass slides. Potentials were measured and are quoted vs. a saturated Hg/HgzSO, reference electrode. The background electrolyte was 0.05 M HzS04 (Analar) in doubly distilled water, in which 50 PM thionine (Fluka, recrystallised) was present for the reversible adsorption and polymer coating experiments. All experiments were carried out in argon purged solutions at room temperature (20°C). Data were processed using a computer programme based on the matrix formulation of polarised light reflection and written in Pascal. The program can produce the best fit to the two (A, JI) or three (A, \k, intensity)‘experimentally measured parameters using a Levenberg-Marquardt based algorithm to obtain the solution for a specified configuration. RESULTS AND DISCUSSION
Values of the optical constants for Pt and PtO at 450 and 632.8 nm, together with the thickness of the latter film under the conditions employed here are necessary background data: we therefore present these first in Table 1. We also required values of the optical constants for reversibly (Langmuirian) adsorbed thionine: these are given in Table 2. TABLE 1 Optical data for Pt and PtOB 632.8 run
450 nm
Pt Pto
n
k
n
k
1.862 2.437
3.709 2.473
2.594 2.938
4.903 3.392
* “PtO” film thickness is 0.497 nm. TABLE 2 Optical constants at 450 nm for Th and Leu adsorbed on Pt a
Leu Th
n
k
1.55 1.45 f 0.05
Ob 0.088 f 0.02
aData obtained assuming L = 0.40 nm. bFixed value.
Measurements
of the thionine polymerisation
process
In this preliminary report, we consider coating for a limited period at medium time resolution (0.2 s). More detailed studies, employing faster data acquisition will be reported in the full paper. Figure 1 shows the A and \k transients accompanying a potential step from 0 V (where a monolayer of reversibly adsorbed Th is present) to 0.82 V (where coating proceeds).
RUN ON Pt IN Th STEPPED FROM 0 TO 042
-541 0
1 50
1 100
1 150
1 200
250
300
L 350
1 400
SAMPLE RUN ON Pt IN Th STEPPED
FROM 0 TO 0.82
I
0 -35
0
50
100
150
200
250
300
350
a0
SAMPLE
Fig. l.(a) Variation of A after a potential step from 0 V to 0.82 V. Data was sampled every 20 ms and the step took place at sample point 100. (b) Variation of 0 as in (a).
We have taken the data for Pt and PtO from Table 1 and for adsorbed Th (as a first approximation for a very thin film) from Table 2 and attempted to model the early time (t < 5 s) ellipsometric behaviour for three possible structures: (i) Pt with a small number of thionine overlayers (0.40, 0.80, 0.12 nm), (ii) Pt with a monolayer of PtO (0.497 nm), (iii) Pt covered by a monolayer of PtO (0.497 nm) plus an overlayer of thionine (0.4 nm). For t < 0, we find case (i) with L = 0.40 nm reproduces the observed A, JI values, as expected. After 2.4 s, the calculated A, \k values using case (ii) are in good agreement with the observed data. At t = 3.6 s, case (iii) gives the best fit. These calculations are summarised in Table 3. One important consequence of this model is that there is an induction time of several seconds before the film begins to grow. This induction time appears to correspond to three consecutive processes: desorption of t.he organic film, formation of “PtO” layer and re-adsorption and, perhaps, chemical reaction of the first layer of thionine with the “PtO”. At this stage we do not have complete access to the time domain to obtain
193 TABLE 3 Calculation of ellipsometric parameters for three possible structures during the early stages of thionine polymer film growth and comparison with observed values Calculated values
Case (i) Case (ii) Case (iii)
Observed values
Time/s
Al”
*I”
-50.84a -51.27 -51.50
35.15a 34.97 34.94
Al”
01”
-50.84 -51.39 -51.49
35.16 34.96 34.97
aFrom a reversal of Table 2 calculations.
perfect fits, firstly because the time resolution is restricted to 0.2 s and secondly because signal/noise problems prevent complete freedom of data point selection. The first problem has received attention and fuller results will be published later. The second problem requires the HeNe laser to be used and we
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194
are constructing a cell with a shorter solution path length to allow reasonable ~~smission of 632.8 nm light. At no time do we find an acceptable fit for Pt covered with more than 0.40 nm (a monolayer) of Th in the absence of an intervening layer of PtO. Consequently, we have modelled the film for t rr 2.5 s as bulk Pt, covered with a monolayer (0.497 nm) of PtO, in turn covered with a layer of thionine. In the c~culations we have used data for Pt and PtO from Table 1, have put nrh equal to 1.47 and have used the experimental A, 9 data to calculate firi, and L as functions of time, for which the results are displayed in Fig. 2a. With the exception of some scatter at early times, the value of kri, remains constant during the first 60 s of coating at 0.11 (+O.Ol). Given that the concentration (and hence e) can only be less than or equal to that of a close packed monolayer this implies an increase in extinction coefficient at 450 um upon pol~e~sation, in accord with visible transmission data [ 261. The film thickness increases approximately linearly with d,t for 1 s < 14 25 s, (Fig. 2b) after which the rate of growth slows. We therefore conclude that film growth is under diffusions control until it is approximately 2 nm thick, after which a limiting thickness (- 3.5 nm for this electrode) is approached more slowly, This will be dealt with in more detail in the full communication, using greater time resolution for the early time data. Measurements on tke polymeric thionine film in background electrolyte In the final type of experiment described here, the coated electrode is removed from the coating solution, rinsed with distilled water and placed in background electrolyte for cyclic voltammetric character&&ion. As the solution is transparent, we chose to employ the He-Ne laser for these experiments. Before analysing data in detail we had to decide whether to model the film as a single (organic) film on Pt or as a duplex (Pt/~~/org~i~) fihn, i.e. we wished to resolve the question as to whether the PtO was reduced at these more negative potentials. The approach used was similar to that in the previous section, except that this time expe~ent~ A, \k values were used to calculate optical and thickness parameters, the plausibility of which could then be examined. Results are given in Table 4 for the reduced form of the film in the absence of PtO, from which it is clear that reasonable values of nL (- 1.5), kL (=O) and L TABLE 4 Optical constants and film thickness calculated for leucothionine (at -0.3 V) polymer film employing a two film model with 0.497 nm PtO (A) and a single film model and a single film model, with no intervening PtO layer (B). A = -40.512” al Yr = 36.261” a
nL kL Llnm
A
B
1.60 Ob 3.75
2.82 Ob 1.12
a Experimental values averaged over 25 paints. bFixed, with other two parameters calculated in Z-point floating fit.
195
(ca. 2-4 nm) cannot simultaneously be obtained according to a single film model. In the following calculations we therefore employ a model with an intervening PtO layer, whose properties are as in Table 1. For the oxidised film, we used a value of 3.75 nm for L, the same value as that found for the reduced film and the experimental A (-42.01”) and j[l (35.62”) values to obtain nr,, = 1.64 and krh = 0.24. The value of nrh is rather higher than for the value found at 450 nm, a fact which undoubtedly reflects the strong optical absorption in the film at 633 nm. The ratio of kTh at 633.8 nm to that measured at 450 nm during film growth (Fig. 2) is -2.1 which may be compared to the ratio of the film extinction coefficients, found by transmission visible spectroscopy, of -1.5. Whether this represents the experimental uncertainty or whether it reflects optical changes at longer times during growth is uncertain. However, it is interesting to note [26 ] that, whilst coating is apparently over within a few minutes, films held at the coating voltage for longer times (-15 min) are more stable. This question is to be pursued further later. We had hoped to use the intensity changes during film cycling to stabilise the extent to which the fihn swelled or shrank during oxidation. Unfo~~ately there is a wide range of (n, k, L) sets of values that differ in reflected intensity very little, and the current experimental accuracy is inadequate to discriminate between them. The good agreement between the ratio of the km values obtained at 633 and 450 nm and that found by absorption indicates, however, that changes in fihn thickness during reduction or oxidation are not large. The value of 1.50 found for the refractive index of the leucothionine form is slightly lower than the value of 1.55 obtained from molar refraction calculations or from cycling the adsorbed monolayer. It is highly likely that this reduction is caused by the presence of solvent in the film, present both to solvate the charged polymer and the associated anions. To summ~se, we have obtained answers to most of the questions posed. (i) PtO is formed during coating. This result is interesting because, firstly, it was known that the threshold potential for polymerisation was roughly that for PtO monolayer formation but not whether this was simply coincidental and, secondly, because attempts at coating solvents in which PtO is not formed (e.g. dry acetonitrile) were unsuccessful [27 3. (ii) The PtO formed is not removed by po~nti~ excursions well negative of the normal reduction peak in the presence of the thionine film. (iii) We have been able to measure the film thickness during growth. (iv) The polymerisation is diffusion controlled in the early stages. (v) The polymer optical constants are rather different from those of the monomer; comparison with adsorbed species implies that the changes are primarily the results of polymerisation rather than surface immobili&ion. In the full paper we shall report the correlation of electrochemical and ellipsometric data for PtO formation on Pt, fast transient (20 ms resolution) and long time data for film growth at different wavelengths, a more definite answer to the redox state/thickness relationship and the results of potential step transients (at the 20 ms level) on polymer coated electrodes.
196 REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
W.J. Albery and A.R. Hiliman, R.S.C. Ann. Rep. C., 78 (1981) 377. R.W. Murray in A.J. Bard (Ed.), Electroanalytical Chemistry, Vol. 13, Marcel Dekker, New York, 1984, p. 191. F.C. Anson, Y.-M. Tsou and J.M. Saveant, J. Electroanal. Chem., 178 (1984) 113. R. Noufi, J. Electrochem. Sot., 130 (1983) 2126. N.S. Lewis, A.B. Bocarsly and M.S. Wrighton, J. Phys. Chem., 84 (1980) 2033. K. Itaya, H. Akahoshi and S. To&ma, J. Electrochem. Sot., 129 (1982) 762. J.A. Cox and P.J. Kulesza, J. Electroanai. Chem., 175 (1984) 105. P.J. Pearce and A.J. Bard, J. Electroanai. Chem., 114 (1980) 89. P. Daum, J.R. Lenhard, D. Rolison and R.W. Murray, J. Phys. Chem., 102 (1980) 4649. N. Oyama, S. Yamaguchi, Y. Niskiki, K. Tokuda, H. Matsuda and F.C. Anson, J. Electroanal. Chem., 139 (1982) 371. M. Majda and L.R. Faulkner, J. Electroanal Chem., 169 (1984) 77. N. Oyama, T. Oksaku, M. Kaneko, K. Sato and H. Matsuda, J. Am. Chem. Sot., 105 (1983) 6003. W.J. Albery, M.G. Boutelle and A.R. Hillman, J. Electroanal. Chem., 182 (1985) 99. G. Inzelt, R.W. Day, J.F. Kinstle and 5.8. Chambers, J. Electroanai. Chem., 161 (1984) 147. R. Denning, J. Electrochem. Sot., 129 (1982) 300C. W.J. Albery, A.R. Hillman, R.E. Hester and K. Hutchinson, J. Chem. Sot. Faraday Trans. I, 80 (1984) 2053. W.J. Aibery, A.R. Hillman, R.G. Egdell and H. Nutton, J. Chem. Sot. Faraday Trans. I, 80 (1984) 111. D.J. Harrison, K.A. Danke and M.S. Wrighton, J. Electroanai. Chem., 163 (1984) 93. P.C. Lacaze and G. Tourillon, J. Chim. Phys. Phys.-Chim. Biol., 76 (1979) 371. F.C. Anson, T. Ohsaka and J.-M. Saveant, J. Phys. Chem., 817 (1983) 640. J.S. Facci, R.H. Schmehl and R.W. Murray, J. Am. Chem. Sot., 104 (1982) 4959. W.J. Albery and A.R. Hiliman, J. Electroanal. Chem., 170 (1984) 27. C.P. Andrieux, J.M. Dumas-Bouchiat and J.-M. Saveant, J. Eiectroanal. Chem., 131 (1982) 1. W.J. Albery, A.W. Foulds, K.J. Hall and A.R. Hillman, J. Electrochem. Sot., 127 (1980) 654. P.R. Trevellick and A. Hamnett, to be submitted. W.J. Albery, M.G. Boutelle, P.J. Colby and A.R. Hillman, J. Electroanal. Chem., 133 (1982) 135. M.G. Boutelle, unpublished work.
189
Preliminary note AN ELLIPSOMETRIC STUDY OF POLYMERIC THIONINE FILMS ON PLATINUM
ANDREW HAMNETT Inorganic Chemistry Laboratory,
South Parks Road, Oxford (Great Britain)
A. ROBERT HILLMAN School of Chemistry, University of Bristol, Cantock’s Close, Bristol (Great Britain) (Received 15th July 1985; in revised form 14th August 1985) INTRODUCTION
The modification of electrode surfaces with polymeric redox films has received considerable attention recently [ 1,2]. Applications to electrocatalysis [ 31, energy conversion [ 41, semiconductor protection [ 51, display devices [ 61 and sensors [ 71 have been described. Characterisation has included electrochemical (cyclic voltarnmetry [8], chronoamperometry [9], chronocoulometry [lo], chronopotentiometry [ 111, normal pulse voltammetry [ 121 and rotating disc electrode [ 131) as well as spectroscopic (UV/visible [ 141, infrared [ 151, Rarnan [16], XPS [ 171, AES [18] and SIMS [19] ) techniques. However, not one of this armoury of techniques permits unambiguous in situ determination of film thickness: this is the question we address here. For example, integration of slow scan rate cyclic voltammograms allows estimation of the total (electroactive) coverage of redox groups, P/m01 cm -‘, but division of this into the concentration, c, and film thickness, L, is not obvious, particularly when swelling factors of up to the order of lo2 have been claimed [20]. Potential step measurements lead to a combination of the effective charge transport diffusion coefficient, D, and the concentration, in the form D “‘c; this essentially gives a relaxation rate constant, D/L’, as P is equal to the product of c and L and is known. Turning to the spectroscopic techniques, UV/visible absorbance data give r (if the extinction coefficient is known) for comparison with the electrochemical estimate, but the problem of the separation of c and L remains. XPS estimation of film thickness using attenuation of substrate photoelectron intensity [ 171 is, of necessity, ex situ, so the value of L does not relate to the electrochemical environment. Physical methods which have been used include step profiling [ 111 and simple micrometer measurement [20], but these are ex situ, are limited (severely in the latter case) in the range of measurable values and may result in film damage or distortion during measurement [20]. The importance of the concentration of redox centres, for which an absolute value is only obtainable if L can be determined, is of both fundamental interest and practical importance. Firstly, the rate of charge transport through the film may be dependent on the site-site separation, i.e. concentration - interestingly in either direction [ 10,211. Secondly, full exploitation of mediated charge 0022-0728/85/$03.30
o 1985 Elsevier Sequoia S.A.
190
transfer-based applications requires a knowledge of immobilised redox centre concentration. The usefulness of a given total loading of redox sites may depend crucially on their distribution, in that there is a trade-off between the rapid charge-transfer/mediation reaction kinetics and poor target solution species permeation/diffusion in compact films with high concentrations of mediator. Only those sites which are readily accessible are effective; further increase in film thickness merely impedes the transport processes and may result in the surface or surface reaction layer cases Sk”, St,, LSk or LSt, (in the notation of Albery [22] ) or R, E or SR (in the notation of Saveant [23] ). For a given value of c, there‘is an optimum value of L for efficient mediation, and the strategy for its determination has been delineated by Albery [22]. It is apparent, then, that L is an important parameter, but that techniques for its direct, in situ, unambiguous, non-destructive measurement are not well developed. The object of this preliminary note is to describe ellipsometric measurements of the thickness and optical properties of a polymeric thionine film [24] on a platinum electrode, firstly, during its growth by anodig electropolymerisation and, secondly, as a function of oxidation state in background electrolyte solution. Specifically, questions we set out to answer were: (i) is PtO formed (as it would be in the background electrolyte at these potentials); (ii) if PtO is formed, is it reduced to Pt at the less anodic potentials used to study the polymer film by cyclic voltammetry; (iii) what is the final film thickness; (iv) what is the growth law; (v) do the optical constants of the polymer differ from those of the monomer; and (vi) is the film thickness redox state dependent? In order to answer these questions we have also obtained results for PtO formation on Pt and for the reversibly (Langmuirian) adsorbed monomeric thionine (Th) species and its reduced form leucothionine (Leu). These will be described in detail in the full paper, and we simply use the results here. EXPERIMENTAL
The ellipsometer was a Rudolph Instruments RR2000 automatic ellipsometer with quartz optics and with a rotating analyser/fixed polariser configuration. No compensator was employed, and intensity, azimuthal angle, cu,and ellipticity, e, could be directly obtained from the analogue computer employed. From Q and E the ellipsometric angles A and \k could be calculated. The instrument was interfaced to an Intertec Superbrain, using digitised output from the intensity, azimuth and ellipticity readouts as described elsewhere [ 251. The period of the rotating analyser is 20 ms and the measurements reported here were made by averaging over a 0.2 s interval. The angle of incidence was 61.33” and the polar&&ion angle employed was 45” throughout. Light sources used were a Hughes 5 mW He-Ne laser with neutral density filter and an Ealing 75 W High Pressure Xe lamp with interference filter, for measurements at 632.8 and 450 rim, respectively.
191
The working electrodes were Pt sputtered on glass slides. Potentials were measured and are quoted vs. a saturated Hg/HgzSO, reference electrode. The background electrolyte was 0.05 M HzS04 (Analar) in doubly distilled water, in which 50 PM thionine (Fluka, recrystallised) was present for the reversible adsorption and polymer coating experiments. All experiments were carried out in argon purged solutions at room temperature (20°C). Data were processed using a computer programme based on the matrix formulation of polarised light reflection and written in Pascal. The program can produce the best fit to the two (A, JI) or three (A, \k, intensity)‘experimentally measured parameters using a Levenberg-Marquardt based algorithm to obtain the solution for a specified configuration. RESULTS AND DISCUSSION
Values of the optical constants for Pt and PtO at 450 and 632.8 nm, together with the thickness of the latter film under the conditions employed here are necessary background data: we therefore present these first in Table 1. We also required values of the optical constants for reversibly (Langmuirian) adsorbed thionine: these are given in Table 2. TABLE 1 Optical data for Pt and PtOB 632.8 run
450 nm
Pt Pto
n
k
n
k
1.862 2.437
3.709 2.473
2.594 2.938
4.903 3.392
* “PtO” film thickness is 0.497 nm. TABLE 2 Optical constants at 450 nm for Th and Leu adsorbed on Pt a
Leu Th
n
k
1.55 1.45 f 0.05
Ob 0.088 f 0.02
aData obtained assuming L = 0.40 nm. bFixed value.
Measurements
of the thionine polymerisation
process
In this preliminary report, we consider coating for a limited period at medium time resolution (0.2 s). More detailed studies, employing faster data acquisition will be reported in the full paper. Figure 1 shows the A and \k transients accompanying a potential step from 0 V (where a monolayer of reversibly adsorbed Th is present) to 0.82 V (where coating proceeds).
RUN ON Pt IN Th STEPPED FROM 0 TO 042
-541 0
1 50
1 100
1 150
1 200
250
300
L 350
1 400
SAMPLE RUN ON Pt IN Th STEPPED
FROM 0 TO 0.82
I
0 -35
0
50
100
150
200
250
300
350
a0
SAMPLE
Fig. l.(a) Variation of A after a potential step from 0 V to 0.82 V. Data was sampled every 20 ms and the step took place at sample point 100. (b) Variation of 0 as in (a).
We have taken the data for Pt and PtO from Table 1 and for adsorbed Th (as a first approximation for a very thin film) from Table 2 and attempted to model the early time (t < 5 s) ellipsometric behaviour for three possible structures: (i) Pt with a small number of thionine overlayers (0.40, 0.80, 0.12 nm), (ii) Pt with a monolayer of PtO (0.497 nm), (iii) Pt covered by a monolayer of PtO (0.497 nm) plus an overlayer of thionine (0.4 nm). For t < 0, we find case (i) with L = 0.40 nm reproduces the observed A, JI values, as expected. After 2.4 s, the calculated A, \k values using case (ii) are in good agreement with the observed data. At t = 3.6 s, case (iii) gives the best fit. These calculations are summarised in Table 3. One important consequence of this model is that there is an induction time of several seconds before the film begins to grow. This induction time appears to correspond to three consecutive processes: desorption of t.he organic film, formation of “PtO” layer and re-adsorption and, perhaps, chemical reaction of the first layer of thionine with the “PtO”. At this stage we do not have complete access to the time domain to obtain
193 TABLE 3 Calculation of ellipsometric parameters for three possible structures during the early stages of thionine polymer film growth and comparison with observed values Calculated values
Case (i) Case (ii) Case (iii)
Observed values
Time/s
Al”
*I”
-50.84a -51.27 -51.50
35.15a 34.97 34.94
Al”
01”
-50.84 -51.39 -51.49
35.16 34.96 34.97
aFrom a reversal of Table 2 calculations.
perfect fits, firstly because the time resolution is restricted to 0.2 s and secondly because signal/noise problems prevent complete freedom of data point selection. The first problem has received attention and fuller results will be published later. The second problem requires the HeNe laser to be used and we
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Fig. 2. (a) Thickness of the thionine film and absorption coefficient calculated from the data in Fig. 1 assuming n = 1.47, plotted against time/s. (b) Variation of film thickness vs. t”*/s”*.
194
are constructing a cell with a shorter solution path length to allow reasonable ~~smission of 632.8 nm light. At no time do we find an acceptable fit for Pt covered with more than 0.40 nm (a monolayer) of Th in the absence of an intervening layer of PtO. Consequently, we have modelled the film for t rr 2.5 s as bulk Pt, covered with a monolayer (0.497 nm) of PtO, in turn covered with a layer of thionine. In the c~culations we have used data for Pt and PtO from Table 1, have put nrh equal to 1.47 and have used the experimental A, 9 data to calculate firi, and L as functions of time, for which the results are displayed in Fig. 2a. With the exception of some scatter at early times, the value of kri, remains constant during the first 60 s of coating at 0.11 (+O.Ol). Given that the concentration (and hence e) can only be less than or equal to that of a close packed monolayer this implies an increase in extinction coefficient at 450 um upon pol~e~sation, in accord with visible transmission data [ 261. The film thickness increases approximately linearly with d,t for 1 s < 14 25 s, (Fig. 2b) after which the rate of growth slows. We therefore conclude that film growth is under diffusions control until it is approximately 2 nm thick, after which a limiting thickness (- 3.5 nm for this electrode) is approached more slowly, This will be dealt with in more detail in the full communication, using greater time resolution for the early time data. Measurements on tke polymeric thionine film in background electrolyte In the final type of experiment described here, the coated electrode is removed from the coating solution, rinsed with distilled water and placed in background electrolyte for cyclic voltammetric character&&ion. As the solution is transparent, we chose to employ the He-Ne laser for these experiments. Before analysing data in detail we had to decide whether to model the film as a single (organic) film on Pt or as a duplex (Pt/~~/org~i~) fihn, i.e. we wished to resolve the question as to whether the PtO was reduced at these more negative potentials. The approach used was similar to that in the previous section, except that this time expe~ent~ A, \k values were used to calculate optical and thickness parameters, the plausibility of which could then be examined. Results are given in Table 4 for the reduced form of the film in the absence of PtO, from which it is clear that reasonable values of nL (- 1.5), kL (=O) and L TABLE 4 Optical constants and film thickness calculated for leucothionine (at -0.3 V) polymer film employing a two film model with 0.497 nm PtO (A) and a single film model and a single film model, with no intervening PtO layer (B). A = -40.512” al Yr = 36.261” a
nL kL Llnm
A
B
1.60 Ob 3.75
2.82 Ob 1.12
a Experimental values averaged over 25 paints. bFixed, with other two parameters calculated in Z-point floating fit.
195
(ca. 2-4 nm) cannot simultaneously be obtained according to a single film model. In the following calculations we therefore employ a model with an intervening PtO layer, whose properties are as in Table 1. For the oxidised film, we used a value of 3.75 nm for L, the same value as that found for the reduced film and the experimental A (-42.01”) and j[l (35.62”) values to obtain nr,, = 1.64 and krh = 0.24. The value of nrh is rather higher than for the value found at 450 nm, a fact which undoubtedly reflects the strong optical absorption in the film at 633 nm. The ratio of kTh at 633.8 nm to that measured at 450 nm during film growth (Fig. 2) is -2.1 which may be compared to the ratio of the film extinction coefficients, found by transmission visible spectroscopy, of -1.5. Whether this represents the experimental uncertainty or whether it reflects optical changes at longer times during growth is uncertain. However, it is interesting to note [26 ] that, whilst coating is apparently over within a few minutes, films held at the coating voltage for longer times (-15 min) are more stable. This question is to be pursued further later. We had hoped to use the intensity changes during film cycling to stabilise the extent to which the fihn swelled or shrank during oxidation. Unfo~~ately there is a wide range of (n, k, L) sets of values that differ in reflected intensity very little, and the current experimental accuracy is inadequate to discriminate between them. The good agreement between the ratio of the km values obtained at 633 and 450 nm and that found by absorption indicates, however, that changes in fihn thickness during reduction or oxidation are not large. The value of 1.50 found for the refractive index of the leucothionine form is slightly lower than the value of 1.55 obtained from molar refraction calculations or from cycling the adsorbed monolayer. It is highly likely that this reduction is caused by the presence of solvent in the film, present both to solvate the charged polymer and the associated anions. To summ~se, we have obtained answers to most of the questions posed. (i) PtO is formed during coating. This result is interesting because, firstly, it was known that the threshold potential for polymerisation was roughly that for PtO monolayer formation but not whether this was simply coincidental and, secondly, because attempts at coating solvents in which PtO is not formed (e.g. dry acetonitrile) were unsuccessful [27 3. (ii) The PtO formed is not removed by po~nti~ excursions well negative of the normal reduction peak in the presence of the thionine film. (iii) We have been able to measure the film thickness during growth. (iv) The polymerisation is diffusion controlled in the early stages. (v) The polymer optical constants are rather different from those of the monomer; comparison with adsorbed species implies that the changes are primarily the results of polymerisation rather than surface immobili&ion. In the full paper we shall report the correlation of electrochemical and ellipsometric data for PtO formation on Pt, fast transient (20 ms resolution) and long time data for film growth at different wavelengths, a more definite answer to the redox state/thickness relationship and the results of potential step transients (at the 20 ms level) on polymer coated electrodes.
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